Methionine-enkephalin and leucine-enkephalin are endogenous pentapeptides that possess opiate activity. The bovine adrenal medulla, which synthesizes and secretes enkephalins, has been chosen for the investigation of the biosynthesis of the enkephalins. Messenger RNA (mRNA) from adrenal medulla has been isolated and used as a template for the cell-free synthesis of enkephalin precursor protein in the wheat germ and rabbit reticulocyte protein-synthesis systems. Specific proteolytic digestion of the adrenal mRNA-dependent proteins yields methionine-enkephalin. The enkephalin precursor protein and enkephalin mRNA are currently being characterized with the goal of understanding the regulation of synthesis of enkephalins, which are thought to be important in pain perception and behavior.